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Many key functionalities of optical frequency combs, such as self-referencing and broad spectral access, rely on coherent supercontinuum generation (SCG). While nanophotonic waveguides have emerged as a compact and power-efficient platform for SCG, their geometric degrees of freedom have not been fully utilized due to the underlying complex nonlinear and noise-dependent stochastic physics. Here, we introduce inverse design to unlock free-form waveguides for coherent SCG. The efficacy of our design is numerically and experimentally demonstrated on Si3N4 waveguides, producing flat and coherent spectra from visible to mid-infrared wavelengths. Our work has direct applications in developing chip-based broadband light sources for spectroscopy, metrology, and sensing across multiple spectral regimes. [ABSTRACT FROM AUTHOR]

Inlet unstart is detrimental to hypersonic airbreathing flights. By unstart, we mean the separation-induced collapse of a supersonic duct flow. The unstart characteristics of a two-dimensional contraction duct with an expansion corner are numerically investigated to develop a universal prediction method and to determine the unstart mechanism. The unstart boundaries under varying contraction angles, entrance heights, and Reynolds numbers are obtained by translating the cowl plate. A self-similar empirical equation of unstart boundary that unifies multiple variables is proposed through similarity analysis. To estimate the unstart boundaries of large-angle ducts, the alternative area ratio is presented. Three types of unstarts are identified according to the distribution of the unstart boundaries and the evolution mechanism of separation region: the long-cowl, transitional, and short-cowl unstart. The long-cowl unstart stems from the mass flow limit at the throat, while the other two unstarts stem from the separation region growth. The wall pressure criteria for unstart are correlated with the dimensionless cowl length and the characteristic separation scale in the critical state to facilitate unstart detection. Moreover, the self-similar compression intensity is proposed to characterize the maximum pressure increase induced by the unit effective area contraction in the started state. [ABSTRACT FROM AUTHOR]

Separation-induced shock reflections in straight ducts are generally considered asymmetric for Mach numbers beyond 2.2, but our experiment shows that this is not always the case. A symmetric shock pattern, a proven outcome of following the free-interaction theory (FIT), is observed to appear in an oscillatory duct flow at a Mach number of 2.47. Interestingly, its existence is restricted to the period when the shocks move forward. Once a full retreat starts, it changes suddenly into an utterly asymmetric style that conforms to the past observation. This behavior indicates that the FIT fundamentally plays a limited role in supersonic duct flows. [ABSTRACT FROM AUTHOR]

Shock-train transitions in simplified curved isolators are carefully studied by simulation. The results show the shock-train behavior is subject to the complex pressure field created by the duct deflection, eventually presenting five modes during a backpressure-varying process. Of them, the most special one is the abrupt shock-train leap. It appears as the leading shocks interact with an adverse pressure gradient and follows a different path after a reversal of the direction the backpressure takes, which causes a shock-train hysteresis. If the curvature increases, the leap phenomenon, together with the related hysteresis, grows in number and intensity. Analysis indicates the background pressure gradients stem from the inherent left-running expansion waves and right-running compression waves. They control alternately the near-wall flow state, provoking the cyclic changes in the pressure gradient sign. Unlike the former, the latter can enhance separation through a positive feedback mechanism, rendering the shock train highly sensitive to backpressure. This is why the leap occurs. Comparing with the previously reported shock-induced leap indicates that there is a marked similarity in their behaviors, suggesting the irrelevance of the occurrence of a leap to the category of incident waves. Nevertheless, a delay in the onset usually follows a compression-wave-typed leap, which reflects that there is a triggering threshold for an incident wave. Given the fact that no local separation is provoked by the compression waves, it is speculated that the threshold should lie below the criterion for causing a separation, as opposed to the impression from the previous research. [ABSTRACT FROM AUTHOR]

We have demonstrated that multi-wavelength Raman and photoluminescence spectroscopies are sufficient to completely characterize the structural properties of ultra-thin hydrogenated diamond-like carbon (DLC:H) films subjected to rapid thermal annealing (RTA, 1 s up to 659 °C) and to resolve the structural differences between films grown by plasma-enhanced chemical vapor deposition, facing target sputtering and filtered cathodic vacuum arc with minute variations in values of mass density, hydrogen content, and sp³ fraction. In order to distinguish unequivocally between films prepared with different density, thickness, and RTA treatment, a new method for analysis of Raman spectra was invented. This newly developed analysis method consisted of plotting the position of the Raman G band of carbon versus its full width at half maximum. Moreover, we studied the passivation of non-radiative recombination centers during RTA by performing measurements of the increase in photoluminescence in conjunction with the analysis of DLC:H networks simulated by molecular dynamics. The results show that dangling bond passivation is primarily a consequence of thermally-induced sp² clustering rather than hydrogen diffusion in the film. [ABSTRACT FROM AUTHOR]

Magnetic and recording properties of antiferromagnetically coupled (AFC) media are studied in comparison to conventional media. AFC media have superior magnetic recording properties compared to conventional media in terms of media signal to noise ratio (S[SUB0]NR) increase and transition pulse width (PW[SUB50]) reduction. Such improvements are most significant for media with (11.0) texture, small underlayer grain size, and low dispersion of the crystallographic texture. AFC media with (10.0) texture do not perform as well. In particular, AFC media have wider PW[SUB50] than conventional media in this orientation. For (11.0) textured media, the large underlayer grain size reduces media SNR improvement of AFC media while exchange field reduction is the major drawback of the deteriorated crystallographic texture. [ABSTRACT FROM AUTHOR]

There have been many studies of the dynamics of a ball rolling on different types of surfaces. Most of these studies have been theoretical, with only a few experimental. We have found that 3D printing offers a novel experimental approach to investigating this topic. In this paper, we use a 3D printer to create four different surfaces and experimentally investigate the dynamics of a ball rolling on these surfaces. Our results are then compared to theory. [ABSTRACT FROM AUTHOR]

TiSiN ultrathin films (10-50 Å) deposited by reactive magnetron sputtering from TixSiy targets were used as anticorrosion overcoats to protect Co-containing recording media. Films' growth, structure, composition, resistance against hydrolysis, and anticorrosion performances were studied by spectroscopy (x-ray reflectivity and diffraction, ellipsometry, x-ray photoelectron spectroscopy, Fourier transform infrared reflection-absorption spectroscopy) and simulated by molecular dynamics (using modified Tersoff-type interatomic interactions). TiSiN ultrathin overcoats were found to be dense amorphous oxynitrides containing Ti-O-Si linkages. The conversion of SiNx into SiOx by hydrolysis was prevented by introducing less than 20 at. % of Ti in the films. Thanks to the formation of Ti-O-Si linkages which densify the films and reduce oxygen diffusion, good corrosion protection of the magnetic media was achieved down to 28 Å TiSiN overcoat thickness. [ABSTRACT FROM AUTHOR]

CuI is used to control the molecular orientation of copper phthalocyanine (CuPc) and modify the anodes in organic solar cells based on CuPc/C60. By introducing a CuI buffer between indium tin oxide and CuPc, the power conversion efficiency is significantly enhanced by a factor of ∼70%. Because of the strong interactions between the CuI and CuPc, the stacking orientation of CuPc molecules is changed, resulting in a ∼65% increase in absorption coefficient, a larger carrier mobility and a smoother film surface. The anode work function is raised by the formation of a dipole layer. [ABSTRACT FROM AUTHOR]

We demonstrated near-infrared organic light-emitting devices employing copper phthalocyanine (CuPc) doped into 4,4′-N,N′-dicarbazole-biphenyl (CBP). Room-temperature electrophos-phorescence was observed at about 1.1 μm due to transitions from the first excited triplet state to the singlet ground state (T1-S0) of CuPc. There existed very weak emission of CBP from undoped devices, and at lower doping concentrations (<=12 wt %) the driving voltages of doped devices were higher than that of undoped devices. The results indicated that Förster [Ann. Physik. (Leipzig) 2, 55 (1948)] and Dexter [J. Chem. Phys. 21, 836 (1953)] energy transfers play a minor role in these devices, and direct charge trapping appears to be the dominant mechanism. [ABSTRACT FROM AUTHOR]

Describes a wavelength-dispersive x-ray fluorescence detector system that can be used to discriminate fluorescent photon energies for practically any incident flux. Use of a Kumakhov polycapillary lens to collimate fluorescent photons from a sample; Resolution of the system; Performance.

We experimentally report the fabrication of deformed silica microtoroids with only one-axial symmetry and very large deformation reaching 18%. The largely deformed microcavities are demonstrated to support ultrahigh-Q resonant modes exceeding 6 × 107, though much more chaos emerges compared with slightly deformed microcavities. The chaotic behavior of rays is beneficial to enhance dynamical tunneling to high-Q whispering-gallery modes, and therefore allows efficient coupling by a free-space laser beam. The largely deformed microcavities hold great potential in applied photonics and fundamental studies. [ABSTRACT FROM AUTHOR]

We report Raman lasing in a slightly deformed silica microcavity using an efficient free-space coupling technique, with an absorbed pump threshold as low as 100 μW in the 1550 nm wavelength band. This ultralow threshold originates from the ultrahigh quality factors of whispering gallery modes, exceeding 108, even though the microcavity is deformed from a circular symmetry. Furthermore, the threshold of free-space coupled Raman lasing is experimentally demonstrated to show a strict inverse square dependence on the intrinsic quality factor of the cavity mode, which agrees well with the theoretical prediction. [ABSTRACT FROM AUTHOR]